High q microwave cavity

ABSTRACT

A low loss microwave transmission structure composed of a section of waveguide within which is concentrically placed an array (one or more) of cylindrical sections of dielectric material with relative dielectric constant greater than four, spaced away from the walls by a significant fraction of the radius (at least of the order of 10 percent) and possessing appreciable discontinuities. In the dielectric loaded sections electromagnetic radiation is propagated primarily within the dielectric so that the losses observed are mainly a function of the dielectric material and are, to first order, insensitive to the wall composition or condition. In this most general form the structure can by synthesized as a low loss filter or equalizer. When operated at frequencies at which the unloaded waveguide is below cutoff a high-Q cavity can be formed. A single section cavity of this type has been used to measure the Q of the low loss high dielectric constant dielectrics. A particularly simple and easily adjustable coaxial input and output structure is available which couples strongly to the cylindrically symmetric TM01 mode, all other propagating modes being greatly disfavored.

[72] inventors [54] llllllGllll Q MlllCROWAVE CllVll'll'Y 6 Claims, 6Drawing lligs.

[52] 111.5. Cl 333/73 W, 333/31, 333/81 B, 333/98, 333/33, 333/83,333/95 [51] lot. 1C1 M0311 9/00 [50] Field oil Smrch 333/73 C,

[56] References Cited UNITED STATES PATENTS 3,475,642 10/1969 Karp etal.333/73 3,271,773 9/1966 Wheeler... 333/95 3,273,085 9/1966 Ash et al...333/83 2,659,870 11/1953 Laemmel 333/8113 2,376,785 5/1945 Krasik 333/81B 1 women 3,028,565 4/1962 Walker 3,413,575 11/1963 CampbellAhS'llM/MCT: A low loss microwave transmission structure composed of asection of waveguide within which is concentrically placed an array (oneor more) of cylindrical sections of dielectric material with relativedielectric constant greater than four, spaced away from the walls by asignificant fraction of the radius (at least of the order of 10 percent)and possessing appreciable discontinuities. In the dielectric loadedsections electromagnetic radiation is propagated primarily within thedielectric so that the losses observed are mainly a function of thedielectric material and are, to first order, insensitive to the wallcomposition or condition. In this most general form the structure can bysynthesized as a low loss filter or equalizer. When operated atfrequencies at which the unloaded waveguide is below cutolT a high-Qcavity can be formed. A single section cavity of this type has been usedto measure the Q of the low loss high dielectric constant dielectrics. Aparticularly simple and easily adjustable coaxial input and outputstructure is available which couples strongly to the cylindricallysymmetric TM. mode, all other propagating modes being greatlydisfavored.

PATENTEUSEP 7|97I I 3503 99 SHEET 1 0r 2 8 I g INNER WALL 0F I WAVEGUIDEQ :2 SURFACE OF 2 DIELECTRIC E o l l l l I l l l l I l l l l l v I00.200 .300 .400 .500 .600 .700 .000 .000 1.000

DISTANCE FROM CENTER IN CENTIMETERS If. GYORG'Y //VVE/V7'ORS RE. JAEGERH. SEIDEI. 8V

PATENTEDSEP H971 1603.899

SHEET 2 BF 2 4 COAXIAL' 4- I TRANSMISSION LINE FLEXIBLE BELLOWSlillllGlill Q li/IICIETGWAVIE CAVllTlt BACKGROUND OF THE INVENTION 1.Field of the Invention The disclosure treats dielectrically loadedcircular waveguide devices capable of low loss signal processing such asfiltering and equalizing. Also treated is the testing of low loss highdielectric constant dielectric materials.

2. Description of the Prior Art The synthesis of reactive transmissionnetworks with predetermined transmission and reflection properties is anold and well-known art. Such devices are used, for instance, as filtersand equalizers in communication systems. In various frequency ranges,the specific embodiments take on widely varying forms. At lowerfrequencies, the elements used are lumped capacitors and inductors. Inother frequency ranges one can use lengths of transmission line anddiscontinuities in the transmission line as the elements of thesynthesis. The discontinuities can be either lumped reactive elementssuch as capacitors and inductors or points at which a transmissionmedium of one characteristic impedance is joined to a trans missionmedium of another characteristic impedance. The latter method is used atmicrowave frequencies, for instance, to form coaxial quarter wavetransformers. This disclosure bears on that part of the art whichconsiders the transmission of electromagnetic radiation throughwaveguide structures. The synthesis of waveguide structures withpredetermined transmission and reflection characteristics is likewise anold and much practiced art. Within this art are filter networks composedof lengths and waveguide within which are placed sections of dielectricmaterial of varying length and dielectric constant either fully orpartially filling the guide. The placing of sections of dielectricwithin waveguide is also a standard method for the testing of theproperties of the dielectric material.

The limitations inherent in the prior art include energy dis sipation inthe waveguide wail due to the finite conductivity of the wall material.This dissipation is a function of the wall material and as such leads tothe dependence of the device properties on the wall material and thecondition of the wall (cg. the wall temperature). Most previously usedstructures which include dielectric loading have the dielectric placedimmediately adjacent to the wall so that expansion and contraction ofthe wall with temperature can cause significant performance variationsdue to large variations of the small air space. Another problem oftenmet is the transmission of spurious modes generated at the junctionbetween the device and the external system.

A class of devices which bears a superficial resemblance to thatdisclosed here is the class of gyromagnetic devices malting use of theFaraday rotation in longitudinally magnetized rods of ferrite withincircular waveguide [Fox et al. Bell System Technical Journal (Jan. 1955)p. (especially pp. 22-28)]. In this ferrite art, however, the verydiscontinuities which form the basis of the filter network synthesisdisclosed here are anathema to the ferrite devices. in fact themicrowave ferrite art includes many techniques, such as the inclusion oflong tapered ends on the ferrite rods, for the diminution of suchdiscontinuities (C. L. Hogan 2,768,354 and E. A. Ohm 2,963,668).

Summary of the Invention The device disclosed here comprises a sectionof circular waveguide within which is concentrically disposed one ormore right circular cylinders ofdielectric material with dielectricconstant greater than approximately four and spaced away from the wallby a distance at least of the order of percent of the guide radius. Ifthe conditions indicated above are met, the energy of theelectromagnetic wave is confined primarily within the dielectricmaterial. If dielectric materials are used which possess low intrinsicloss, a low loss structure is produced, since a relatively small amountof the transmitted field contacts the waveguide wall. Such devices canbe designed by an appropriate synthesis procedure, using thediscontinuities between sections, to form signal processing networks,such as filters or equalizers.

Useful device classes include the operation of such a device at afrequency at which one or more: ofthe loaded sections are in acutoffcondition. Since the fields in a cutoff section are exponentiallydecreasing, these cutoff sections can be entirely free from dielectric.A test fixture for the measurement of the loss properties of dielectricmaterials has been constructed using a single cylinder of dielectricmaterial and operating with the unloaded ends in a cutoff condition.This forms a high Q resonator. All device classes disclosed here sharethe proparty that they can be observed in reflection as well as intransmission. Thus, useful devices can be envisioned which are connectedto the external system at only one end, the other end being suitablyterminated.

However, the above considerations hold for all propagating modes iftransformation to a coaxial geometry is necessary and transmission isthe TM mode a simple and efficient transducer is produced by introducinga probe which is coaxial to the waveguide. The transformation from thewaveguide to the coaxial geometry takes place with very little spuriousmode generation because of the great similarity between the TM and thecoaxial field geometries.

Brief Description of the Drawing FIG. l is a partially sectionedperspective view of a general transmission structure, within the scopeof the disclosure, containing an array of dielectric cylinders ofvarying dielectric constant, length and diameter;

FIG. 2 is a curve, derived by calculation, showing the square of themagnetic field (ordinate) as a function of the distance from the axis ofthe structure (abscissa) for the situation in which a cylinder of radius0.8 centimeter of the material of relative dielectric constant 10 issituated within a waveguide of radius one centimeter and the transmittedradiation is 10" H23,

FIG. 3 is a partially sectioned perspective view of a two-section high Qband-pass filter;

FIG. I is a partially sectioned plane view of a transducer from awaveguide to a coaxial geometry including a length of flexible bellowsbetween the outer conductor of the coaxial cable and the waveguide foreasy adjustment of probe penetration;

FIG. 5 is a partially sectioned plane view of an exemplary cavityincorporating the transducer of FIG. 4; and

FIG. (6 is a partially sectioned plane view of a device incorporating atapered transformer section between a device of the disclosed class andan external waveguide system.

Definition of Some Important Terms Loaded in this disclosure loadedimplies containing. A section of the waveguide which is dielectricallyloaded," contains a cylinder of dielectric material.

Cutoff In this disclosure cutoffimplies a nonpropagating condition. Ageneral expression representing a wave propagating in the z direction isA A e 'f f where w is the angullar frequency of the wave and k is thepropagation constant.

When the transmission is taking place in a waveguide structure thereexists a frequency, dependent upon the particular waveguide geometry andknown as the cutoff frequency," below which the propagation constant,It, becomes imaginary. Below this frequency the above expression becomesA A e c which represents an exponentially decaying wave. The wave, thenis decreasing in the z, direction and not freely propagating. Belowcutoff" implies operation at a frequency such that the wave isexponentially decay mg.

Detailed Description of the Invention 1. General Considerations FIG. 1shows a general structure of this class comprising a portion of acircular waveguide 10 within which is concentrically disposed an arrayof dielectric cylinders 11, I2, 13, I4, 18, 19, each having a differentdielectric constant, diameter and length. Each of the cylinders shouldhave a relative dielectric constant at least of the order of four and bespaced away from the waveguide wall by a least of the order of 10percent of the waveguide radius. In addition the space between thedielectric cylinders and the metallic wall must be below cutoff in thetransverse direction. If these conditions are met the electromagneticenergy is largely confined within the dielectric cylinder and anyeffects due to the metallic wall are small.

A complete device would also include means for introducingelectromagnetic radiation 33 and 34. Such means might be the simplejunction to a circular waveguide wherein the external system has causedelectromagnetic radiation to be propagated, a tapered transformationsection 60, a more complex transducer such as the coaxial probe 42 to bedescribed below or any other transducer known in the art. A device ofthis general class can be designed to perform reactive functions such asfiltering and equalization by suitable choice of cylinder length,diameter and dielectric constant. The change of diameter and dielectricconstant from one section to the next must be such as to produce asignificant discontinuity. It is considered that if the product of thecylinder diameter and the square root of the dielectric constant changesby less than ten percent within a distance equal to one quarter of thecylinder diameter the discontinuity will not be significant and amarginally useful device will be produced.

2. Choice ofStructure Parameters The teaching of this disclosurecomprises the following qualitative picture: The electromagnetic energyis largely confined within the dielectric cylinder; since the spacebetween the cylinder and the wall is transversely cut off, the fieldsdecrease exponentially away from the cylinder decreasing to a low valueat the wall; since only this small field contacts the wall, effects(such as loss) due to the wall are minimal. Thus, one must includeenough dielectric material to contain a major part of the field energyyet not allow the cylinder to approach the wall too closely.

A solution of Maxwells equation for this geometry yields more specificinformation concerning the choice of optimum parameters. For every setof physical parameters (dielectric constant and frequency of operation)an optimum set of geometric parameters (waveguide diameter and cylinderdiameter) can be determined. For material near the low end of theacceptable range, the choice of optimum parameters is important for therealization of improved performance.

With a material having a dielectric constant of 4 and a dissipationfactor of 2 l0 and with propagation in the TM mode there is as muchdissipation in the walls of the waveguide as there is within thedielectric material at the optimum geometry so that the improvementsafforded by this construction becomes marginal for materials ofdielectric constant less than 4.

For dielectric materials of relative dielectric constant of the order ofor greater, the field energy is well contained within the dielectric anda choice of the ratio between the diameters of the cylinder and thewaveguide is not critical as long as the restrictions mentioned aboveare observed. Typical structures operating at 10" Hz. have a diameterratio of 0.8 and a waveguide diameter of 2 centimeters. The magneticfield distribution within such a device containing a dielectric materialwith a relative dielectric constant of I0 is illustrated in FIG. 2.These solutions can be extended to other frequencies by realizing that,in the low loss regime, Maxwells equations scale with frequency so longas all elements of the structure are scaled properly.

Since the walls of the waveguide contribute in only a minor way to theproperties of devices of the disclosed class, the device properties arerelatively insensitive to the composition or conditions of the waveguidewalls. The wall material can then be chosen for properties other thanhigh conductivity (e.g. structural strength, corrosion resistance orthermal stability). In addition the variation of metallic conductivitywith temperature becomes unimportant.

3. Use of Cutoff Sections A section of waveguide which is cut offrepresents a greater discontinuity than the junction between twodifferent dielectrics. If cutoff sections are included in the structure,transmission characteristics can be synthesized which vary much morerapidly with frequency. Typical structures using cutoff sections arehigh Q bandpass filters. The cutoff sections can be entirely free fromdielectric material and not significantly increase the loss of thestructure because of the exponential decrease of field strength withinthe cutoff region. The absence ofmaterial in the cutoff section allowsthe easy variation of the coupling between adjacent dielectric sections.

FIG. 3 shows a typical structure which is a two-section band-passfilter. This filter can be designed to have either a maximally flat orChebishev response. The spacing between the ends of the dielectriccylinders 31, 32 and the structures which couple to the device 33, 34 ischosen to give the required energy coupling to the external circuit 35,36.

4. Tapered Transformer Input When a device of the disclosed type isadjoined to an external waveguide system it may be necessary to minimizethe junction discontinuity. This can be done, for instance, by theinclusion of a tapered transformer section having a tapered dielectriccylinder 61 for impedance matching and filtering such as is depicted inFIG. 6.

5. Coaxial Probe When it is required to transform to a coaxial geometryth use of the TM mode is particularly advantageous. The simpleintroduction of a probe 42 coaxial to the waveguide 40, as illustratedin FIG. 4, forms a transducer which produces no coupling tononcylindrically symmetric modes and very little coupling to any modesother than TM because of the great similarity between the TM and thecoaxial field geometry.

This figure also shows that the probe penetration can be easily adjustedby the inclusion of a flexible bellows 43 between the waveguide and theouter conductor of the external coaxial transmission line 44.

6. Dielectric Loss Test Fixture The teaching of this disclosure hasallowed the construction of a test fixture (see FIG. 5) for themeasurement of the dielectric loss of high dielectric constant material,comprising a circular waveguide structure 50 with a single dielectriccylinder 51 (FIG. 5 illustrates a more complex device containing twospaced cylinders 51) and two coaxial transducers 52, 53, and 54 operatedat a frequency for which the waveguide is cut off outside of thedielectrically loaded section. This test fixture is significantly betterthan the structures reported in W. B. Westphal, Tech. Rep. 182, Lab. forInsulation Research, M.I.T., Oct. 1963 and S. B. Cohn et al. IEEETransaction NUT-1409 (Sept. 1966) which are considered to berepresentative of the prior art. By considering a material having adielectric constant of 10 and a dissipation of 2X10 one can derive afigure of merit for this structure. A meaningful figure of merit is theratio of dielectric loss to metal wall loss expressed as R/D where "Rcharacterizes cavity Q in the presence of wall losses only and "Drepresents a similar quantity for the dielectric losses. For the cavityand material described above, this figure of merit has a value of 24showing that the effect of wall loss is very small.

7. The Composition ofthe Ambient All of the discussions above haveassumed that all spaces outside of the dielectric cylinders are filledby vacuum or a gaseous medium whose relative dielectric constant isclose to one. However, all of the above considerations hold if the saidspaces are filled by some other dielectric material so long as itsdielectric constant is at least four times lower than the smallestdielectric constant of the dielectric cylinders.

What we claim is:

. An electromagnetic wave conducting structure comprisa portion ofcircular electrically conductive waveguide;

at least two right circular cylindrical sections of a dielectricmaterial, which are not resonant at a frequency of intended use,concentrically disposed therein; and

means for launching into said portion of circular waveguide anelectromagnetic wave of such frequency that said wave decays essentiallyexponentially in a radial direction in the space between saidcylindrical sections and said waveguide, said means terminating at leastone end of said portion of circular waveguide characterized in that thesaid cylinders are spaced away from the wall of the said waveguide by atleast of the order of IO percent of the waveguide radius, that therelative dielectric constants of the said cylinders are at least of theorder of four and that the structure contains discontinuities ofmagnitude such that the product of each dielectric cylinder diameter andthe square root of its dielectric constant changes from one section tothe next by more than l0 percent within a distance equal to one quarterof the cylinder diameter.

2. A device of claim 1 in which said means for launching is a structurecomprising a coaxial probe jointed to the center con ductor of a coaxialtransmission line whose outer conductor is electrically joined to thesaid waveguide.

34 A device of claim 1 including at least one section cutoff in thedirection of propagation for the mode employed 4. A device of claim 2wherein the said outer conductor is joined to the said waveguide wall bymeans of a flexible metallic bellows.

5. A device of claim 1 wherein at least one of said cylindrical sectionsis terminated, at least at one end, by a tapered transformer sectioncomprising a tapered section of dielectric material.

6. A device of claim 1 in which said electromagnetic wave is in the TMmode.

1. An electromagnetic wave conducting structure comprising: a. a portionof circular electrically conductive waveguide; b. at least two rightcircular cylindrical sections of a dielectric material, which are notresonant at a frequency of intended use, concentrically disposedtherein; and c. means for launching into said portion of circularwaveguide an electromagnetic wave of such frequency that said wavedecays essentially exponentially in a radial direction in the spacebetween said cylindrical sections and said waveguide, said meansterminating at least one end of said portion of circular waveguidecharacterized in that the said cylinders are spaced away from the wallof the said waveguide by at least of the order of 10 percent of thewaveguide radius, that the relative dielectric constants of the saidcylinders are at least of the order of four and that the structurecontains discontinuities of magnitude such that the product of eachdielectric cylinder diameter and the square root of its dielectricconstant changes from one section to the next by more than 10 percentwithin a distance equal to one quarter of the cylinder diameter.
 2. Adevice of claim 1 in which said means for launching is a structurecomprising a coaxial probe jointed to the center conductor of a coaxialtransmission line whose outer conductor is electrically joined to thesaid waveguide.
 3. A device of claim 1 including at least one sectioncut off in the direction of propagation for the mode employed.
 4. Adevice of claim 2 wherein the said outer conductor is joined to the saidwaveguide wall by means of a flexible metallic bellows.
 5. A device ofclaim 1 wherein at least one of said cylindrical sections is terminated,at least at one end, by a tapered transformer section comprising atapered section of dielectric material.
 6. A device of claim 1 in whichsaid electromagnetic wave is in the TM01 mode.